1. Field of the Invention
The present invention relates to an optical disc playback apparatus for optically reading and/or playing back data recorded in a disc-shaped recording medium (hereinafter referred to as an optical disc), such as a CD (Compact Disc) or a DVD (Digital Versatile Disc). More specifically, the present invention relates to tracking servo control of an optical disc playback apparatus.
2. Description of the Related Art
FIG. 7 is a block diagram of an optical disc playback apparatus. The optical disc playback apparatus shown in FIG. 7 includes an optical pickup 10, an RF amplifier 12, a signal processor 14, a digital-to-analog converter (DAC) 16, a low-pass filter (LPF) 18, a gain adjuster 20, a servo equalizer 22, a hold equalizer 24, a switch 26, a driver 30, and a defect detection circuit 28. The optical pickup 10 irradiates an optical disc with light, and converts the reflected light into an electrical signal. The RF amplifier 12 generates a readout signal (RF signal) and a tracking error (TE) signal based on the signal from the optical pickup 10. The signal processor 14 performs processing, such as decoding, on the RF signal from the RF amplifier 12. The DAC 16 converts the signal from the signal processor 14 into an analog signal. The LPF 18 removes the high-frequency component of the signal from the DAC 16, and outputs the resulting audio signal. The gain adjuster 20 adjusts the gain of the TE signal from the RF amplifier 12. The servo equalizer 22 corrects for the frequency characteristics of the gain-adjusted signal. The hold equalizer 24 holds the signal from the servo equalizer 22. The switch 26 selects from the output signals of the servo equalizer 22 and the hold equalizer 24, and supplies the selected output signal as a tracking driving (TD) signal to the driver 30. The driver 30 controls a driving motor etc., of the optical pickup 10 based on the TD signal from the switch 26. The defect detection circuit 28 detects any defects on the optical disc, including scratches, stains, and black spots, and outputs a DFCT signal indicating a detected defect. According to the DFCT signal output from the defect detection circuit 28, the switch 26 supplies the hold signal of the hold equalizer 24 to the driver 30.
FIG. 8 is a diagram showing a three-beam signal detection system of the optical pickup 10. A main beam M at the center is used for reading signals and for focusing the beam onto a track. Two sub-beams S1 and S2 are deviated by about a quarter of the track pitch in the track line direction. A differential circuit 32 determines the difference between the electrical signals obtained from the sub-beams S1 and S2, and the RF amplifier 12 generates the TE signal based on the difference.
As shown in FIG. 9A, an RF signal (a) drops at the position of a defect on the optical disc. The defect detection circuit 28 detects this drop, and generates a DFCT signal (b) having a pulse width corresponding to the period of time during which the RF signal falls. When a defect of the optical disc is detected, a TE signal (d) suffers large amplitude turbulence, and becomes undesirable. Therefore, if a defect is detected, the switch 26 outputs, as a TD signal (c), the hold signal held before the defect is detected to the driver 30. This hold signal is substantially constant, and provides stable tracking servo for the driver 30. Thus, the main beam M of the optical pickup can properly follow a target track.
For example, Japanese Unexamined Patent Application Publication No. 2000-90467 discloses a servo device having a tracking hold function for any defects on an optical disc.
However, an optical disc playback apparatus of the related art has the following problems. When the optical disc apparatus is in an environment susceptible to shock or vibration, e.g., in a vehicle, an external force caused by shock or vibration affects an optical pickup. In such an environment, when a defective disc is played back, lens oscillation occurs due to the acceleration component caused by vibration although the level of the TD signal is held.
FIGS. 10A to 10C are signal waveforms in playing back a defective disc in a two-dimensional vibration environment. In FIGS. 10A to 10C, “TE signal (without vibration)” indicates a TE signal in a case where the optical disc playback apparatus is used in a non-vibration environment, and “TE signal (with vibration)” indicates a TE signal in a case where the optical disc playback apparatus is used in a vibration environment. The TE signal on the plus side indicates that the lens of the pickup is shifted to the outer circumference, and the TE signal on the minus side indicates that the lens is shifted to the inner circumference. In a vibration environment, the TE signal exhibits a waveform in which oscillation levels overlap. In passing through defects, the farther from the target track the lens position is, the larger turbulence the TE signal suffers at the on-track time after the level holding for the defects.
The TE signal is at the most negative/minus point (i.e., the lens is at the innermost position) at defect detecting time T1, and is at the most positive/plus point (i.e., the lens is at the outermost position) at defect detecting time T3. At defect detecting time T2, the lens is substantially in the center position. Thus, the TE signal suffers large amplitude turbulence at the defect detecting times T1 and T3.
In larger amplitude levels, as shown in FIG. 10B, if time T4 at which the lens position is far from the target track coincides with the time at which a defect is passed through, the TE signal has an excessive large amplitude level, and the servo becomes out of control.
FIG. 10C shows waveforms of the TE signal when an optical disc having no defects is played back in a vibration environment. Although servo is out of control at time T5 at which the lens position is far from the target track, the servo function is still achievable even in a relatively large vibration environment, compared to playback of a defective optical disc. Therefore, in playing back a defective optical disc, a problem occurs in that servo is susceptible to vibration.